1. Field of the Invention
This invention relates to a real-image viewfinder for a photographic camera, and more particularly to a real-image viewfinder for a variable-exposure-size photographic camera in which the field of view of the viewfinder is changed by an improved mechanism.
2. Description of the Prior Art
There has been an increasing demand for miniaturization of cameras from the viewpoint of popularization of cameras, and various miniaturized cameras have been developed and put into practice. Though there has been known a mini size camera using a special small size film, there is a problem that resolution of the picture deteriorates and sharpness of the image deteriorates as the film size decreases and at the same time such a special small size film is less available. Accordingly, it is preferred that the camera be miniaturized while using common films.
Recently, a real-image viewfinder has come into wide use, and it is preferred that even a camera having a real-image viewfinder be as small as possible. In the real-image viewfinder, an inverted image of an object is once formed through a convex object lens and then the inverted image is further inverted into an erected image through an erecting prism system. The erected image is magnified through a convex eyepiece. In some cameras with a real-image viewfinder, a mask having a field frame is positioned in the image surface so that the field frame is viewed through the finder window.
Recently there has been put into practice a variable-exposure-size photographic camera in which the exposure size (the size and/or the aspect ratio of the picture) can be switched between normal size and panoramic size. Such a variable-exposure-size photographic camera is provided with a pair of masks having different field frames, one for the normal size and the other for the panoramic size, and a field frame switching mechanism selectively positions masks for the panoramic size in an operative position in the image surface or in a retracted position away from the light bundle passing through the optical system of the viewfinder while the mask for the normal size is kept stationary in the image surface.
FIGS. 11A and 11B respectively show examples of conventional field frame switching mechanisms. In the example shown in FIG. 11A, a panoramic-size mask 30 is positioned beside an image surface 13 as shown by the solid line while the normal-size mask (not shown) is kept stationary in the image surface 13 when the normal-size exposure is selected, and is inserted in the image surface 13 over the normal-size mask as shown by the broken line when the panoramic-size exposure is selected. In the example shown in FIG. 11B, the panoramic-size mask comprises upper and lower halves 31A and 31B, and the upper and lower halves 31A and 31B are moved toward each other into the image surface 13 when the panoramic-size exposure is selected while are kept above and below the image surface 13 when the normal-size exposure is selected.
However, since the panoramic-size mask should be as close to the image surface as possible when it is in the operative position, the mechanism for driving the panoramic-size mask is very complicated in the conventional field frame switching mechanisms.